Mood disorders have traditionally been considered to be neurochemical disorders, but new evidence demonstrates impairments of structural plasticity and cellular resilience in the central nervous system. Recent preclinical and clinical studies have shown that signaling pathways involved in regulating cell survival and death are long-term targets for the actions of mood stabilizers. Lithium and valproate are common mood stabilizers for bipolar disorder and are shown to indirectly regulate cell pathways, including CREB, BDNF, Bcl-2, and MAP kinases, which may stimulate some of their delayed long-term beneficial effects. Specifically, lithium and valproate upregulate Bcl-2 in vivo and in vitro, preventing neuronal cell death through blocking the mitochondrial program for apoptosis. Mitochondrial dysfunction has been recently reported in Alzheimers' and Parkinson's diseases and it is not clear if similar mitochondrial abnormalities occur in bipolar disorder. The purpose of this ongoing study is to investigate the effects of mood stabilizers on various aspects of mitochondrial function. Rat primary cortical neurons and human neuroblastoma SH-SY5Y cells were treated chronically with valproate and lithium at therapeutically relevant concentrations. Mitochondrial protein levels were determined with western blot analysis; respiratory activity was measured using the Oxygraph system. Western blot analysis showed upregulation of porin (a voltage dependent anion selective channel) and anti-apoptotic proteins Bcl-2 and Bcl-x/L in the mitochondrial fraction of cells treated with lithium and valproate. Oxygraph measurements demonstrated that lithium and valproate increased respiratory rate both time- and dose-dependently. It is postulated that Bcl-xL and Bcl-2 may act to influence the properties of other outer membrane proteins to maintain their ability to pass complex anions. Recently it was reported that Bcl-xL can interact with porin and regulate its gating properties in vitro. It has also been shown that Bcl-2 over-expressing cells result in an increased mitochondrial volume and structural complexity. Since Bcl-2 is a key regulator of cell survival, we have used SiRNAs (small interfering RNAs) to knock down bcl-2 levels and determine the consequences of this manipulation on the biochemical effects of mood stabiliziers. SiRNAs are small double-strand RNAs, 21-22 nucleotides in length that trigger the degradation of cognate RNA and induce the knock-down of a specific gene expression in mammalian cells. In this study, we investigated the role of Bcl-2 as a regulator of mitochondrial function, through the use of siRNA knockdown techniques. Bcl-2 siRNA/pSilencer significantly knocked down Bcl-2 gene expression, resulting in ~40% reductions in Bcl-2 protein levels in human neuroblastoma SH-SY5Y cells. 3 different dyes were used to study the effects of lithium and valproate (VPA) on quantity of mitochondria, mitochondrial membrane potential, mitochondrial oxidation and generation of free radicals (ROS) in neuronal cells. Mitotracker green becomes fluorescent in the lipid environment of mitochondria regardless of the membrane potential; JC-1 accumulates in proportion to the mitochondrial membrane potential; Mitotracker red oxidizes to a fluorescent product once inside the mitochondria and 2',7'-dichlorofluorescin diacetate (H2DCFDA) for assessing oxidative stress by staining ROS. Confocal microscope study showed that chronic lithium treatment increased mitochondrial membrane potential of the SH-SY5Y cells. The knockdown of Bcl-2 gene in SH-SY5Y cells was accompanied by clear reductions in mitochondrial oxidation and significantly reduced Bcl-2-protein levels. These results suggest that mood stabilizers enhance mitochondrial function through Bcl-2 upregulation; in view of the critical role of mitochondria in regulating short & long-term plasticity, these effects may be critical to the therapeutic effects. To further investigate the chronic effects of mood-stabilizers on mitochondrial function in vivo, adult Wistar rats were treated with lithium or VPA chow for 4 weeks, then challenged with neurotoxin methamphetamine (5mg/kg, i.p, 4 times in 8h). We found that chronic lithium and VPA treatment attenuated methamphetamine-induced decrease of Bcl-2 and increase of Bax in rat frontal cortex mitochondrial fraction. VPA treatment preserved rat frontal cortex mitochondrial electron transport complex IV-- cytochrome C oxidase activity, while lithium pretreatment significantly prevented methamphetamine-induced hyperthermia and high-dose toxic mortality. These novel results suggest that lithium and valproate may exert some of their long-term effects on neuroplasticity and cellular resilience via hitherto underappreciated effects on mitochondrial proteins and mitochondrial function.